1. Field of the Invention
The invention relates to a photoelectric position measuring device for measuring the relative position of two objects which are movable with respect to each other, wherein a graduation scale connected with one object is scanned by means of a scanning unit connected with the other object, wherein two diffraction beams of one order emanating from a first graduation area meet in a second graduation area under the same associated diffraction angles.
2. Description of the Prior Art
Position measuring devices in the form of either a length-measuring device or an angle-measuring device are employed for measuring the relative position of two objects, for example, a machine tool with respect to a workpiece to be processed by the machine tool.
For example, U.S. Pat. No. 4,792,678 based upon German Letters Patent DE-PS 36 33 574 discloses a photoelectric angle-measuring device for measuring the angular position of two objects which can be turned towards each other. The incremental angular graduation of a rotatable graduated scale connected with one object is scanned by a scanning unit connected with the other object. The scanning unit comprises a light source, a deflecting prism and a photoelectric cell. A beam of light emanating from the light source impacts a first angular graduation area of the angular graduation which is in the form of a phase grid. The beam of light is split by the angular graduation into a positive diffraction beam of the first order with a positive diffraction angle and a negative diffraction beam of the first order with a negative diffraction angle. After having been twice deflected by the deflection prism, the two diffraction beams combine again under two equal associated diffraction angles in a second angular graduation area, which is located diametrically opposite from the first angular graduation area. The two diffraction beams interfere in the second angular graduation area and impact a photoelectric cell to generate a scanning signal, from which the measured values regarding the angular position of the objects are derived. This double scanning of the angular graduation in two angular graduation areas, located diametrically opposite from each other, by means of diffraction of light beams eliminates eccentricity errors which are caused by the eccentricity between the angular graduation and the rotating shaft of the graduated scale which are practically always present. Inaccurate measurements result from such eccentricity errors.
Errors are also introduced by the wobbling of the graduation disk caused by the not exact vertical mounting of the graduated scale on the rotating shaft. Increased demands made on the accuracy of measurement, however, no longer permit the tolerance of wobbling errors in angular measurements. Because of the wobbling of the graduated scale, the meeting point of the two diffraction beams in the second angular graduation area wanders in tangential and radial directions. The result is that the impact point of the light beam emanating from the light source in the first angular graduation area and the meeting point of the two diffraction beams in the second angular graduation area are no longer located on the same radius. Because the grid constant of the incremental angular graduation depends on the radius, different grid constants are therefore being scanned in the two angular graduation areas resulting in inaccurate measurements.
The same problems correspondingly occur in the form of tip errors in length measuring devices using the same scanning principle. In the case of rotation of the linear graduation around an axis crosswise to the direction of measurement (for example in connection with a wavy graduation), the meeting point of the two diffraction beams is displaced in the direction of measurement by a certain amount, which is directly introduced into the measured result in the form of a measurement error.
Therefore, it is an object of the present invention to compensate for wobble errors or tip errors occurring in photoelectric position measuring devices in a simple way by providing an optical deflection element so that accurate measurement results are obtained.